There is a high demand for longer chain α-olefins, especially even numbered α-olefins such as 1-hexene and 1-octene. 1-hexene and 1-octene are used, amongst others, as co-monomers in polyethylene production where they serve as plasticizers e.g. in the preparation of linear low density-polyethylene.
One method of producing olefins is through an olefin metathesis reaction. A disadvantage of this type of reaction is that it is difficult to control the reaction to produce only one specific olefin, and the majority of olefins produced by this process are internal olefins. Metathesis reactions are accordingly not very suitable for preparing α-olefins such as 1-hexene or 1-octene. One type of metathesis reaction, namely ethenolysis between an internal olefin and ethylene, can potentially yield α-olefins, but the technology suffers from equilibrium and selectivity limitations. Furthermore, ethenolysis of an internal olefin would result in an olefin with a shorter chain than the starting internal olefin.
1-Hexene can also be produced by the trimerisation of ethylene, however C4, C8 and C10 impurities are also produced.
WO03/024910 discloses a process of increasing the carbon chain length of olefinic compounds, including α-olefins, such that 1-pentene can be converted to 1-hexene and 1-heptene can be converted to 1-octene. The process comprises the steps of:—                providing a starting olefinic compound and subjecting it to hydroformylation to produce an aldehyde and/or alcohol with an increased carbon chain length compared to the starting olefinic compound;        optionally, hydrogenating the aldehyde that forms during the hydroformylation reaction to convert it to an alcohol which has an increased carbon chain length compared to the starting olefinic compound; and        subjecting the alcohol with the increased carbon chain length to dehydration to produce an olefinic compound with an increased carbon chain length compared to the starting olefinic compound.        
Although α-olefins such as 1-hexene and 1-octene can be produced by the process of WO03/024910, the process suffers from several disadvantages. Firstly, as can be seen from the Examples in WO03/024910, the alcohols formed during the hydroformylation step have a relatively low linearity (i.e. not more than about 88%). Low linearity in the alcohols formed during the hydroformylation step renders the process undesirable from an economical point of view. Further, in the process of WO03/024910 it is desirable to remove branched alcohols from the hydroformylation product before dehydration in order to achieve an acceptable purity of the final α-olefin product.
It would therefore be desirable to provide a process for producing α-olefins which obviates the above disadvantages.
It has now been found by the present inventors that by using a specially selected hydroformylation catalyst the linearity of the alcohol produced in the hydroformylation step can be improved. In addition, the use of the specially selected hydroformylation catalyst leads to an overall improvement in the purity of the final α-olefin product.